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The second-lowest layer (layer 2)
in the OSI Reference Model stack is the data link layer, often
abbreviated DLL (though that abbreviation has other meanings
as well in the computer world). The data link layer, also sometimes
just called the link layer, is where many wired and wireless
local area networking (LAN) technologies primarily function. For example,
Ethernet, Token Ring, FDDI and 802.11 (wireless Ethernet
or Wi-Fi) are all sometimes called data link layer
technologies. The set of devices connected at the data link layer
is what is commonly considered a simple network,
as opposed to an internetwork.

The data link layer is often conceptually
divided into two sublayers: logical link control (LLC) and media
access control (MAC). This split is based on the architecture used
in the IEEE 802 Project, which is the IEEE working group responsible
for creating the standards that define many networking technologies
(including all of the ones I mentioned above except FDDI). By separating
LLC and MAC functions, interoperability of different network technologies
is made easier, as explained in our earlier discussion of networking
model concepts.

Data Link Layer Functions

The following are the key tasks performed
at the data link layer:

Logical Link Control (LLC): Logical link
control refers to the functions required for the establishment and control
of logical links between local devices on a network. As mentioned above,
this is usually considered a DLL sublayer; it provides services to the
network layer above it and hides the rest of the details of the data
link layer to allow different technologies to work seamlessly with the
higher layers. Most local area networking technologies use the IEEE
802.2 LLC protocol.

Media Access Control (MAC): This refers
to the procedures used by devices to control access to the network medium.
Since many networks use a shared medium (such as a single network cable,
or a series of cables that are electrically connected into a single
virtual medium) it is necessary to have rules for managing the medium
to avoid conflicts. For example. Ethernet uses the CSMA/CD method of
media access control, while Token Ring uses token passing.

Data Framing: The data link layer is responsible
for the final encapsulation of higher-level messages into frames
that are sent over the network at the physical layer.

Addressing: The data link layer is the
lowest layer in the OSI model that is concerned with addressing: labeling
information with a particular destination location. Each device on a
network has a unique number, usually called a hardware address
or MAC address, that is used by the data link layer protocol
to ensure that data intended for a specific machine gets to it properly.

Error Detection and Handling: The data
link layer handles errors that occur at the lower levels of the network
stack. For example, a cyclic redundancy check (CRC) field is often employed
to allow the station receiving data to detect if it was received correctly.

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